1. Field of the Invention
The present invention relates to an electrophotographic image printing apparatus such as a copying machine, printer, or FAX machine and, more particularly, to an image printing apparatus having a control unit which uniformly controls the rotational speed of an image carrier such as a photosensitive drum.
2. Description of the Prior Art
In an electrophotographic image printing apparatus, a toner image is formed on an image carrier comprising a rotating photosensitive drum, photosensitive belt, or the like, and the formed toner image is directly or indirectly transferred and fixed on an image recording sheet. In forming an image, an image exposure unit performs image exposure on the image carrier uniformly charged by a charging unit, thereby forming a latent image. In forming a latent image, however, when the peripheral speed of the image carrier which rotates at a uniform rotational speed varies, the formed image distorts. A tandem color image printing apparatus prints a color image on an image recording sheet by superimposing monochrome images formed by a plurality of monochrome image printing units. Essential requirements for obtaining high-quality color images are that the image carriers in the respective monochrome image printing units must rotate at the same speed without any speed unevenness.
Various control methods using various speed detecting units have been proposed for speed control on photosensitive drums. When the angular speed of a photosensitive drum is to be controlled to a constant speed in real time, a rotational speed control method of performing rotational speed control with an angular speed detecting unit using an encoder is used.
For example, for a tandem color image printing apparatus, a feedforward (repetitive) control technique is available as a technique of suppressing a reproducible variation per rotation as in the case with an image carrier such as a photosensitive drum, and a method of controlling rotation variations with respect to a control target rotational position by using rotational speed data at the same position in the cycle one cycle (rotation) before the control target rotational position and its near position has been proposed. See, for example, Japanese Unexamined Patent Publication No. 2000-162941 (patent reference 1).
In the above feedforward (repetitive) control, however, when an uncontrollable accidental variation occurs, the variation is stored as a controlled variable at the position. Even when no accidental variation occurs in the subsequent rotation cycles, a control unit acts to suppress this variation state, adversely affecting subsequent rotations.